1. Field of the Invention
The invention relates in general to a handover method, and more particularly to a handover method used in a wireless communication system.
2. Description of the Related Art
Referring to FIG. 1, a diagram of a wireless communication system is shown. A wireless communication system 100, such as a GSM system, a CDMA system or a wireless LAN, includes a cell 102 of a first base station 101 and a cell 112 of a second base station 111. The wireless communication system 100 provides wireless Internet access or wireless communication to a mobile device 120. The mobile device 120 can be a mobile phone or a portable device with wireless Internet access such as a mobile phone or a personal digital assistant (PDA) for instance.
Suppose initially, the mobile device 120 is at position A and communicates with the first base station 101. The first base station 101 transmits a first signal, while the second base station 111 transmits a second signal. Position D is the farthest position that the first signal emitted by the first base station 101 can reach. The mobile device 120 can no longer receive the first signal from the first base station 101 beyond the position D. In other words, the signal power required for the mobile device 120 to receive the first signal from the first base station 101 at position D is a predetermined power value. Therefore, the mobile device 120 must be handed over to the second base station 111 from the first base station 101 before reaching position D, lest data might be lost.
The conventional handover method is exemplified below. During the movement of the mobile device 120, the mobile device 120 compares the power of the first signal emitted by the first base station 101 and measured the power of the second signal emitted by the second base station 111. When the mobile device 120 is at position P, the power from the first base station 101 is smaller than the power from the second base station; the mobile device 120 is handed over to perform communication via the base station 112.
However, if the decision of handover is determined according to the signal power of the base station received by the mobile device 120 is larger than a fixed value, unnecessary handovers would occur and cause extra burden to the wireless communication system. For example, the received power of the first signal emitted by the first base station still suffices to maintain a good communication when the mobile device 120 may move around position P for a while. If the mobile device is handed over simply because the received power of the second signal from the second base station is slightly larger, there would be too many handovers which cause extra burden to the wireless communication system. If the mobile device 120 is handed over when the received power of the second signal emitted by the second base station is larger than a fixed value, data loss would occur because the mobile device 120 moves too fast. The mobile device 120 might have no enough time to process the handoff procedures and data transmission.